OTDRs are used, for example, for testing glass fibers. The OTDR includes an optical receiver having an avalanche photodiode or a PIN diode as a receiving element, and a transimpedance amplifier which converts the photocurrent delivered by the receiving diode into a voltage. The principle of such a circuit is described in Hewlett-Packard Journal, December 1988, pages 26 to 28.
A significant problem with such circuits is the circuit's behavior on overloading by large optical pulses. Assuming a conversion factor between optical input power and output voltage of 1000 V/mW, which is usual for transimpedance amplifiers in the measurement technology, and further assuming a supply voltage of 12 V, saturation of the amplifier is reached at an optical power of as low as 12.mu.W.
Upon overloading, such transimpedance amplifiers have very long relaxation times, typically up to 10.mu.s, until the operating point is exactly readjusted. This behavior is particularly disadvantageous when measuring optical backscatter since very low signal levels (about 1nW) have to be measured, while at the same time it is possible that very large optical pulses (up to 1 mW) occur resulting from reflections at plugs.
It would be beneficial to provide an OTDR in which, after receiving a large reflected pulse, the time for reaching a value of about 0.01% deviation from the operating point be no greater than 30 ns. Moreover, it would be useful to provide a transimpedance amplifier, or more generally a current-voltage converter, with improved behavior on overloading, without changing the bandwidth and other essential parameters of the amplifier. Neither of these objectives is achieved by known transimpedance amplifiers.
Accordingly, it is an object of the present invention to provide a current-voltage converter of the type useful in optical time domain reflectometry which is minimally influenced by large input pulses. A further object of the invention is to provide such an amplifier which in addition has a bandwidth comparable to that of presently existing amplifiers of the same type. The present invention achieves these goals.